Postage rates depend upon thickness of the envelope. The United States Postage Service (USPS) requires additional postage for an envelope exceeding ¼″ thick when the envelope does not exceed 6⅛″ by 11½″ (letter size), or if the envelope is larger than 6⅛″ by 11½″ (flat size) additional postage is required when the flat exceeds ¾″ in thickness. Any package over 12″ by 15″ is considered a parcel and parcel rates apply.
The USPS does not accept rate measurement based upon the use of a slot as a gauge, if the slot is horizontal or even vertical, when force can be applied to the envelope to pass it through the slot. The slot gauge is not acceptable even if use thereof does not apply any force to the envelope to get it into or through the slot gauge.